Whatever "bubble" means, it should probably only be used as a description of an abrupt crash in retrospect, otherwise we're always in a non-falsifiable bubble state. Saying "We're in a bubble" really just means colloquially "I think this market segment is overvalued", probably not much more than that.
No, that's just its defining characteristic. You can "tell" by other signs too -- but they could be misleading.
>Whatever "bubble" means, it should probably only be used as a description of an abrupt crash in retrospect, otherwise we're always in a non-falsifiable bubble state.
That's the case with every phenomenon that depends in the final result for its definition. E.g. is an act X "beneficial"? We might have some heuristics, but we can only certainly know from its effects after it is completed.
So it doesn't matter whether people repeat prophecies or doom for a long time or not -- unless it's actually a bubble, it won't ever burst. At worse it will start declining slowly.
That's why the arguments like "it hasn't burst all those years so it's not a bubble" don't get it either.
Being a bubble is not something that has to do with duration (whether it lasts for a long time or not) -- it has to do with the non-linear effects of the burst.
Of course it the burst never comes, or it's instead some gradual decline, then it's not a bubble.